The invention relates to a circuit suitable for controlling a load in an electrical system having a relatively positive, relatively high-voltage supply terminal and a relatively negative common return terminal, the circuit being adapted for connection between the high-voltage supply terminal and one terminal of the load, another terminal of the load being connected to the return terminal, the circuit comprising an electrically conductive semiconductor substrate carrying a high-voltage semiconductor output device in which conduction is predominantly by electrons, the substrate comprising a region of the output device for connection to the high-voltage supply terminal, the substrate also carrying at least one relatively low-voltage semiconductor device which is connected for operation from two low-voltage supply terminals.
Such circuits, in which a power or high voltage semiconductor device has low power devices integrated alongside, are described in R. S. Wrathall et al: "Integrated circuits for the control of high power", IEEE IEDM Technical Digest 1983, pages 408-411. The low power circuitry provided typically includes control logic and monitoring circuits closely associated with the power device, and may afford surge protection, thermal protection, short-circuit protection, current sensing and/or other features, all on-chip, giving rise to the terms "intelligent" or "smart" power chip.
High-voltage smart power chips have applications in automotive ignition circuits, or as drivers for high-voltage electroluminescent displays for example. In other applications, particularly where inductive elements are present, high-voltage breakdown capability is required even though the nominal supply voltages are relatively low. For example, in automotive electrical systems, where the supply voltage is nominally +12 or +24 volts d.c., the supply often carries spikes of 50 volts and more, of either polarity. The low power circuitry, which may for example comprise bipolar, MOS or CMOS circuitry to perform logic and/or analog functions, is typically operated from a 5 to 12 volts supply which is referenced to zero volts (or the vehicle chassis), and perhaps derived by a regulator, also on-chip. For reasons of space and expense, it is not practicable to fabricate all of this low power circuitry to have the high breakdown voltages that the power device is designed for and so high-voltage isolation is provided between the low-voltage devices and the high-voltage devices.
Devices in which conduction of the main current is predominantly by electrons include n-channel MOS transistors and n-p-n bipolar transistors. Such devices are preferred in high power applications because the mobility of electrons is superior to that of holes, which means that devices with n-type conduction can carry more current per unit area than otherwise identical devices in which conduction is by holes. If the output device is of a type having four semiconductor regions, for example a thyristor or COMFET (insulated-gate rectifier), then connection of the positive supply terminal to the substrate may be made via a region of p-type semiconductor to form the anode of such a device. Such a region may typically be provided as a highly doped layer across the back of an n-type substrate. The electrical connection to the predominant n-type region of the substrate is maintained however, since the p-n junction thus formed will be forward biassed.
The power semiconductor is often constructed so as to employ the substrate itself as one terminal. The chip can then be bonded to a metal heat sink, which also forms a terminal of the integrated circuit. This is often desirable for devices such as power MOSFETs and bipolar devices, because higher currents may then be handled using so-called vertical structures. This increases the problem of isolation when providing the low-voltage `intelligent` circuitry on-chip because in many applications the terminal that is the substrate of the chip is one which must be connected to the high-voltage supply terminal. This is particularly the case in automotive applications, since it is frequently necessary to provide switches for headlamps, screen heaters, etc. between the positive battery terminal and a load which is permanently connected to the chassis (common return) of the vehicle. Such switches will be referred to hereinafter as `high-side switches`. A similar problem arises in a.c. circuits or when using the `switch` as part of a full-bridge circuit, for reversible motor control. The low-voltage devices, which conventionally operate with reference to the ground (or chassis) voltage must therefore be isolated from the substrate on which they are formed.
In the known high-side power chips the integrated low power circuitry has been isolated by the provision of special diffusions or barrier layers of differing conductivity types to interpose high-breakdown-voltage reverse-biased p-n junctions between the substrate and the low power devices or by the provision of dielectric layers. While technically satisfactory, these isolation techniques are expensive, involving one or (usually) more additional, and often special processing steps.